This invention relates to a loudspeaker protection circuit which interrupts the supply of the amplifier output signal to the loudspeaker when the circuit senses a condition leading to a potential loudspeaker failure caused by an improper input signal or a fault within the amplifier.
There are two principal conditions which can produce loudspeaker damage. The first of these is thermal overload caused by the application of excessive power to the loudspeaker for a sufficient period of time to cause the voice coil to burn out. The second of these is excursion overload caused by application of a signal of a frequency and amplitude to cause excessive motion of the speaker cone, thus producing physical damage. At low frequencies, excursion overload occurs at lower power input than is necessary for thermal overload.
There are a number of known techniques for protecting loudspeakers from one or both of these conditions. The simplest of these techniques is to incorporate a circuit breaker or fuse between the amplifier and the loudspeaker. This can provide a good measure of protection against thermal overload, but not for excursion overload, since lower power levels can also cause cone damage. In view of the fact that both frequency and power level are important, distinguishing a desirable signal from an undersirable signal on the basis of amplitude alone is not a viable technique.
Electronic circuits are known which sense the DC or low frequency signals applied to the loudspeaker and operate a protection circuit if they exceed a certain predetermined limit.
These circuits may utilize simple filters which remove the higher frequency components of the applied signal or more complex filters which have a subsonic cutoff frequency so that the circuit responds only to those frequencies which are very near DC. The relatively simple filter designs suffer from the disadvantage that they will respond to frequencies which are within the audio frequency spectrum, unless their cutoff frequency is made very low. A very low cutoff frequency creates a very slow response time to fault conditions which apply DC to the loudspeaker; typical response times for such filters being a half second or more, thus increasing the risk of damage. More complex filters can improve this response time but require more components which tends to decrease the reliability of the protection circuit.